1. Field of the Invention
The present invention relates in general to a semiconductor memory device such as SRAM and in particular to a structure of a sense amplifier which amplifies data on bit lines of the semiconductor memory device.
2. Description of Related Art
FIG. 1 shows a circuit diagram of SRAM according to a related art. Each memory cell (cell) is connected to a paired bit line BL and EL (/BL or BLbar). These arrayed memory cells are called memory cell array. In this related art, there are eight paired bit lines (BL less than 0 greater than  and /BL less than 0 greater than  to BL less than 7 greater than  and /BL less than 7 greater than ), and each bit line is connected to a PMOS transistor T as a transfer gate. Each bit line is connected to a local sense amplifier 1 via the PMOS transistor T.
The circuit of FIG. 1 requires a plurality of local sense amplifiers 1 since these memory cells are arranged horizontally. The plurality of local sense amplifiers 1 is connected to a data bus 2 for outputting data stored in these cells.
The data bus 2 is connected to a global sense amplifier 3 connected an output buffer 4. The global sense amplifier 3 is generally arranged close to the output buffer 4.
The each memory cell (cell) is connected to a word line 5, and the each PMOS transistor T are connected to a control line 6.
The method of outputting data from this SRAM is explained below. Data stored in a memory cell connected to word line 5 selected according to row address are transferred to the bit line (for example BL less than 0 greater than  and /BL less than 0 greater than  of FIG. 1). And then, only the data in the bit line selected based on column address on the control line 6 is inputted to the local sense amplifier 1 via transfer gate T (for example, the first and second PMOS transistor from the left of the FIG. 1). The output data of the local sense amplifier 1 is outputted to the global sense amplifier 3 via the data bus 2. The global sense amplifier 3 amplifies the output data to output to the output buffer 4. The output buffer 4 may output the data to outside of this device.
In this semiconductor memory device of the related art, the static capacitance of data bus 2 increases very much in order to connect a large number of local sense amplifiers 1 (for example, sixty four (64)) to the data bus 2. Therefore, when the local sense amplifier 1 senses the data, it needs to charge a large static capacitance. Then, larger power is required in case of shorten senses time is required. Namely, there is a problem that the sense time is lengthened in case of less power supply to the local sense amplifier 1.
Here, it is possible to shorten the charging time of the static capacitance of the outputting data, if the power of local sense amplifier 1 is further increased. In general, however, the upper limit of electric power in the LSI chip is often predetermined. Therefore, there is a limit in increasing power of local sense amplifier 1 for faster outputting.
Accordingly, present invention is to provide a novel semiconductor memory device capable of improving sense time for high-speed data outputting, while increasing power at the sensing is restrained.
A semiconductor memory of the present invention comprises a plurality of paired bit lines; a plurality of memory cells; a plurality of first local sense amplifiers connected to at least one bit line of the paired bit lines, for amplifying data output from the bit lines; a plurality of second local sense amplifiers connected to the first local amplifiers, for amplifying data output from the first local sense amplifiers; and a data bus connected to the second local sense amplifiers, for transferring data output from the second local sense amplifiers.